Home > DeveloperSection > Interviews > What's the difference between a primary key and a unique key?

Posted on    January-16-2015 10:42 PM

 MSSQL Server SQL Server 2012 
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Chintoo Semi

Total Post:79

Posted on    January-16-2015 11:42 PM

Both primary key and unique key enforces uniqueness of the column on which they are defined. But by default primary key creates a clustered index on the column, where are unique creates a nonclustered index by default. Another major difference is that, primary key doesn't allow NULLs, but unique key allows one NULL only.

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